1. Field of the Invention
The invention relates generally to sputtering of materials. In particular, the invention relates to a magnetron creating a magnetic field to enhance sputtering.
2. Background Art
Sputtering, alternatively called physical vapor deposition (PVD), is the most prevalent method of depositing layers of metals and related materials in the fabrication of semiconductor integrated circuits. The semiconductor industry typically uses DC magnetron sputtering in which a wafer to be sputter deposited is placed in opposition to a metal target across a plasma reactor chamber filled with an argon working gas. The target is biased sufficiently negatively with respect to the chamber that the argon is excited into a plasma. The positively charged argon ions are strongly accelerated toward the negatively biased target and sputter metal atoms from it. The metal atoms dislodged from the target fall at least in part on the wafer and are deposited in a layer thereon.
In metal sputtering, the target or at least its inner surface has substantially the same metallic composition as that desired for the sputter deposited layer, for example, aluminum, copper, titanium, tantalum, tungsten, etc. In reactive sputtering, a chemically reactive gas such as nitrogen is additionally supplied into the chamber and the reactive gas reacts with sputtered metal atoms at the wafer surface to deposit a metal compound on the wafer, such as the refractory metal nitrides TiN, TaN, WN. The refractory nitrides are particularly useful as barrier layers between a dielectric and a later sputtered metal layer, and a layer of the associated refractory metal is sputtered first onto the dielectric to serve as a glue layer promoting adhesion of the metal to the dielectric. Accordingly, it is often advantageous to use the same sputter reactor to deposit a bilayer liner of, for example, Ti/TiN, Ta/TaN, or W/WN. Sputtering is also used to coat the sides of a via hole with a thin copper seed layer that nucleates and provides an electrode for subsequent filling of copper into the hole by electrochemical plating (ECP).
However, for advanced integrated circuits, sputtering suffers from the problem that sputter deposition, as described above, involves a fundamentally ballistic process of transporting sputtered atoms from the target to the wafer. In the process, the sputtered atoms are emitted in a broad pattern about the normal to the target. Such a distribution is ill suited to filling narrow holes, such as via holes extending through an inter-level dielectric layer separating two layers of metallization. Such via holes in advanced devices may have aspect ratios of 3:1 and greater. A broad sputtering pattern causes the top of the high aspect-ratio hole to close before the bottom is filled. As a result, voids form in the sputtered via metallization. Similarly, the sputtered liner layers tend to be much thicker at the top of the via hole than at the bottom.
One method of adapting sputtering to deep hole filling, as well as to other applications, is self-ionized plasma (SIP) sputtering, as disclosed by Fu in U.S. Pat. No. 6,183,614, by Chiang et al. in U.S. patent application, Ser. No. 09/546,798, filed Oct. 8, 1999 and now issued as U.S. Pat. No. 6,306,265, and by Fu et al. in U.S. patent application, Ser. No. 09/564,798, pending filed Apr. 11, 2000, all three incorporated herein by reference in their entireties. SIP sputtering allows a significant fraction of the sputtered atoms to be ionized using a somewhat conventional sputtering reactor. The charged sputtered metal ions can be electrically attracted into narrow via holes in the wafer. Furthermore, the sputtered metal ions can in part be attracted back to the target to further sputter the target, thereby allowing the pressure of the argon working gas to be significantly decreased. In the case of copper, it is possible to eliminate the need for the argon working gas after the plasma has been ignited in a process called sustained self-sputtering (SSS).
Most SIP magnetrons include an outer pole of a first magnetic polarity arranged in a closed band surrounding an inner pole of an opposed, second magnetic polarity. Further, the total magnetic flux integrated over the area of the outer pole, also called the total magnetic intensity, is significantly larger than that of the inner pole, preferably by a factor of at least 1.5. The resultant unbalanced magnetron produces a magnetic field component projecting from the outer pole towards the wafer before that component bends back to close on the back of the outer pole. Such a projecting magnetic field has several advantages. It extends the plasma farther away from the target; it guides ionized sputtered particles towards the wafer; and, it reduces plasma electron leakage to the chamber walls.
An SIP magnetron should be relatively small in order to increase the effective plasma power density, which for a fixed target power increases inversely with the area of the magnetron. A prevalent design for a SIP magnetron includes an outer pole having a generally triangular shape with the triangular base of the outer pole positioned near the periphery and the triangular apex located near the central axis of the chamber about which the magnetron is rotated to provide angularly uniform sputtering of the target.
Such a simply shaped magnetron is relatively constrained in the ease of optimizing its design for the various requirements of a magnetron intended for commercial usage, such as sputtering uniformity, high sputtering rate, projection of the magnetic field, and low operating pressure. A further problem is that sputtered material tends to redeposit at the periphery of the target where the magnetic field tends to be less. The redeposited material is not effectively resputtered and has a crystallography different than the original target. As a result, the redeposited material grows in thickness and tends to flake off, thereby producing deleterious particles which significantly reduce chip yield.
One technique for increasing the peripheral magnetic field and hence preventing redeposition from accumulating includes moving the inner pole closer to the outer pole near the target periphery. However, this change pushes the high-density plasma closer to the grounded or floating shield near the target edge. The proximity of the plasma to such an electrically biased shield has been observed to cause problems with the stability of the plasma as the magnetron sweeps the plasma around the target periphery. Arcing to the shields may occur, which also produces particles. The plasma may shrink away from the shields or even collapse in some situations, obviously severely impacting deposition uniformity and yield. SIP sputtering typically relies upon high target powers, usually identified by high target voltage, for example, around xe2x88x92500 to xe2x88x92800 VDC and requiring powers of 30 kW or more. For reasons of economy, the size of the power supply is selected to operate near its limit. With the plasma positioned close to the shields, plasma fluctuations have been observed which temporarily raise the target voltage to xe2x88x921000 VDC, sufficient to bum out the typically used power supplies.
Accordingly, it is desired to provide an unbalanced magnetron having additional design freedom. It is also desired to increase the sputtering rate near the target periphery without bringing the principal magnetic field too close to the periphery. It is further desired to improve the uniformity of sputter deposition.
A further problem arises from the desire to reduce the argon pressure so that the plasma is barely supported and is operating in conditions close to extinguishment. Such a plasma is unstable. Even if it does not extinguish, it may change in intensity and distribution, effects which degrade the desired uniformity of sputter deposition.
An unbalanced magnetron useful for DC sputtering having a nested magnetron and an auxiliary magnet. The magnetron is rotated about a center of the target being sputtered.
In one aspect of the invention, the auxiliary magnet is a horizontally magnetized planar magnet placed between the inner and outer poles on the side of the inner pole closer to the target periphery. The poles of the planar magnet may be chosen to reinforce the pole of the inner and outer magnetic poles closer to the target. Such a planar auxiliary magnet increases the sputtering rate near the outer target periphery.
In another aspect of the invention, the auxiliary magnet is a vertically magnetized magnet placed on an opposite side of the rotation axis from the major portion of the nested magnetron. Preferably, the polarity of the auxiliary magnet corresponds to the polarity of the outer magnetic pole. Such a vertical auxiliary magnet increases the vertical magnetic field adjacent the wafer.
The two aspects may be combined in a single magnetron.
The nested magnetron has an inner pole of one magnetic polarity surrounded by an outer pole of another polarity and including an auxiliary magnet and is preferably an unbalanced magnetron in which the total magnetic flux of the outer pole is significantly greater than that of the inner pole, for example, in a ratio of at least 1.5. The nested magnetron may have a generally triangular outer shape with the axis of rotation near the apex of the triangle and the base of the triangle near the target periphery.
The auxiliary magnet may be used to optimize various portions of the magnetic field distribution. One effect of the auxiliary magnet may be to increase the unbalance of the magnetic field distribution.